1. Field of the Invention
The present invention relates to a method for manufacturing an electric-resistance-welded steel pipe which is used under a wet hydrogen sulfide environment.
2. Description of the Related Arts
Steel pipes which transfer petroleum and natural gas including hydrogen sulfide are susceptible to hydrogen-induced cracking (hereinafter referred to simply as "HIC") and sulfide stress corrosion cracking (hereinafter referred to simply as "SSCC"). The presumed mechanism of HIC generation is that, under an environment with the presence of hydrogen sulfide, hydrogen generated by the corrosion of steel surface becomes hydrogen atoms, which atoms easily penetrate into the steel body, and the atomic hydrogen generates bubbles in the vicinity of non-metallic inclusion in the steel, and the pressure of generated bubbles induces cracks. The cracks further propagate through a non-uniform region such as a segregated zone and a ferrite-pearite interface to develop a large crack.
Examined Japanese Patent Publication No. 57-16184 discloses a steel material having a good HIC resistance property which is used for a pipeline. The steel material consists essentially of 0.03 to 0.20 wt. % C, 0.01 to 0.50 wt. % Si, 0.03 wt. % or less P, 0.002 to 0.005 S, 0.01 to 0.10 wt. % Al, more than 1.2 wt. % and 2 wt. % or less Mn, oxygen which is inevitably contained in the steel material, 50 ppm or less Ca which is added in accordance with oxygen content [% O] and sulfur content [% S], and the balance being Fe and inevitable impurities. The oxygen content [% O], the sulfur content [% S] and the Ca content [% Ca] satisfy the following equation: EQU [% Ca].times.{1-97[% O]}/[% S].gtoreq.1.0.
A steel slab cast by a continuous casting process or a steel ingot of 400 mm or more in thickness, which has the above-mentioned composition, is subjected to rolling. In this prior art, the shape control of inclusion by Ca addition prevents the inclusion from becoming the starting point of cracks.
On the other hand, SSCC is a cracking that occurs during the stress application, and the phenomenon differs from the mechanism of HIC. Both types of cracking are, however, common in that the hydrogen which enters into the steel triggers the cracks from the point of non-metallic inclusion. Accordingly, reduction of non-metallic inclusion improves the SSCC resistance.
For the hot-rolled steel sheet as the base material of electric-resistance-welded steel pipe, both the resistance to HIC and the resistance to SSCC are necessary to be improved by the above-described countermeasures.
The characteristics of electric-resistance-welded steel pipe are, however, not improved solely by the improvement of base material. The presumed reason is that inclusion consisting mainly of fine oxide exists on a joining face at the electroseaming section and that hydrogen concentrates to the face to bring the inclusion as the starting point of cracks.
Unexamined Japanese Patent Publication No. 63-241116 discloses a method for a electric-resistance welded steel pipe having a excellent SSCC resistance property. A steel strip is prepared. The steel strip consists essentially of 0.15 to 0.35 wt. % C, 0.1 to 0.8 wt. % Si, 0.2 to 0.5 wt. % Mn, 0.2 to 0.6 wt. % Cr, 0.05 to 0.25 wt. % Mo, 0.01 to 0.06 wt. % Al, at least one element selected from 0.01 to 0.15 wt. % Zr and 0.001 to 0.15 Hf, at least one element selected from the group consisting of 0.01 to 0.15 wt. % Nb, 0.01 to 0.15 wt. % V and 0.01 to 0.15 wt. % Ti, the balance being Fe and inevitable impurities. P and S are contained as impurities. The amount of P is 0.02 wt. % or less and the amount of S is 0.005 wt. % or less. A ratio of Mo wt. % to Cr wt. %, that is, Mo (wt. %)/Cr (wt. %) is from 0.2 to 0.4. An electroseaming section of the strip is heated in a non-oxidizing atmosphere. The electroseaming section is electrically welded so that an upset or a difference between outer circumferential length of the pipe before and after welding may be from 25 to 125% of wall thickness. Then the produced pipe is heated to a temperature of 920.degree. to 1020.degree. C., quenched from the temperature and tempered.
Actually, however, there has not been developed a method that has an excellent shielding property and is durable for continuous operation.